The present invention relates to an electron beam system designed for use in microfabrication of large scale integrated circuit patterns. More particularly, the invention concerns an electron beam system which is capable of the formation and projection of entire characters or portions thereof, such as those characters to be written in large scale integrated circuit patterns of repetitive nature, i.e., magnetic bubble memory patterns.
Electron beam columns have been adapted widely for use in systems for the microfabrication of large scale integrated semiconductor circuits. Such columns have particular utility in the writing of selected patterns on semiconductor wafers, i.e., in projecting the patterns upon selected areas of photoresist which are then developed to form the photoresist masks extensively used in a wide variety of operations during IC fabrication. The function of any such system is to write a given pattern over a given area with a specified charge density and with adequate edge resolution in the shortest time possible. In cases where compatibility with other lithography systems is required, absolute deflection accuracy and repeatability are essential. For very small patterns it becomes necessary to alter the charge density at different points along the pattern to compensate for proximity effects. In addition, for multi-level patterns it is essential to be able to overlay the successive layers of the pattern with satisfactory accuracy.
In view of the above requirements various electron beam lithography systems have been designed and utilized. The typical electron beam system utilized in connection with such integrated circuit microfabrication may include an electron beam source, condenser lenses, alignment stages, demagnification lens stages, a projection lens, a deflection unit and a target area arranged in well known fashion. For example, U.S. Pat. No. 3,644,700 issued Feb. 22, 1972 to Kruppa et al., describes a typical electron beam column. Other electron beam columns and components thereof are described in U.S. Pat. No. 3,949,228 to Ryan and U.S. Pat. No. 3,984,678 to Loeffier et al.
One widely used approach in the design of electron beam columns used in microfabrication is a round beam imaging approach. In this approach, an electron beam is demagnified to form a small focused image of the electron gun crossover point. The spot profile is approximately Gaussian in nature. The demagnification is adjusted so that the spot diameter is smaller than the smallest pattern line width required to be written. Each pattern element is then written by moving the beam sequentially from point to point over the pattern element until the entire written area is filled in.
If necessary, the dwell time at each point on the pattern element can be adjusted to provide proximity effect corrections. A round beam imaging system can be utilized either in a vector scanning mode or in a line by line or orthogonal scanning mode. Such systems have an advantage in that they can be utilized to write both orthogonal and diagonal patterns. However, the Gaussian spot is a small pattern diameter and, therefore, the time requirements in writing of typical semiconductor patterns are prohibitive to the efficient use of such systems for manufacturing. Moreover, the pattern data involved in recreating a given set of semiconductor images with a Gaussian spot is extremely massive. Accordingly, the data processing capabilities of such systems are expensive.
Other electron beam columns have been adapted for microfabrication of semiconductors utilizing a fixed shape beam image. For example, the Kruppa patent mentioned hereinbefore describes one such system. Thus, a square beam system exposes a number of image points in parallel and gains an equivalent factor in thruput over Gaussian spot type systems. In such schemes the electron beam is focused to provide a demagnified image of an aperture called the beam shaping aperture. The beam shaping aperture is square and such systems are particularly useful in fabricating or following patterns by sequentially filling in squares of the pattern area. The size of the focused square spot is generally chosen to be the same as the minimum pattern line that is required and the optical system is designed so that the edge resolution of the spot is considerably less than this. Each pattern element is written by moving the shaped beam in discrete jumps so that the pattern is written as a series of squares.
The square beam imaging systems have certain advantages over the traditional Gaussian round beam systems, as are set forth in detail in the publication "New Imaging and Deflection Concepts for Proforming Microfabrication Systems", by H. C. Pfeiffer, Journal of Vacuum Science and Technology, December 1975, Vol. 12, No. 6, pages 1170-1173. However, the square beam imaging systems have certain disadvantages in that they require a handling capability for a great deal of pattern information or data; and in addition, the use of such square shaped beam systems in an efficient manner is limited to recreation of beam patterns having a large percentage of orthogonally oriented pattern outlines. Thus, patterns involving a large number of diagonal outlines are difficult to reproduce.
Other imaging concepts have been utilized in electron beam columns; for example, in copending application Ser. No. 771,235, for Method and Apparatus for Forming a Variable Size Electron Beam, H. C. Pfeiffer, P. M. Ryan and E. V. Weber, filed Feb. 23, 1977, and assigned to the assignee of the present application. The referenced application describes a system utilizing an extension of the fixed shape beam imaging concept. However, in the method described two beam shaping apertures are used, placed at conjugate planes, and a deflection system is placed between the two apertures. With the deflection system switched off the second square aperture is fully illuminated, thereby resulting in a square focus spot of maximum dimensions. When the deflection system is energized, the illumination of any portion of the second square aperture can be blanked off producing a rectangular or square spot of any desired size, up to the maximum, containing the same current density as the original square spot.
Since the size of the spot can be arbitrarily reduced the maximum dimensions of the spot can be larger than the minimum line width of the pattern. A pattern can then be filled in utilizing various combinations of square and rectangular beam images whereby an improved thruput system is attained. In addition, the variable shape beam system provides a capability of writing lines whose widths are not in normal multiples of the basic spot widths without the need for double exposure. Also, it permits use of a framing technique in which the central region of the pattern element is covered with large squares and the perimeter is written with thin lines to reduce electron interaction effect and thereby maintain good edge resolution.
Despite the advantages of the variable shape spot system it is more efficiently used with circuit patterns having a large percentage of orthogonal orientations since it is not easily adapted for writing or handling diagonal data patterns. Moreover, it is not susceptible of easy use with circuit patterns, such as those required in fabrication of magnetic bubble memory circuits, which embody a large number of repetitive elements, such as the typical I and T patterns used for bubble memories.
Accordingly, a need exists in the prior art for an improved electron beam column which would enable fast writing of large scale integration circuit patterns of repetitive nature.